A DC/DC converter has been put to practical use as an output voltage controller which outputs a constant voltage based on supply of a DC input voltage. A DC/DC converter is a small, lightweight, high-efficiency DC power supply which uses a semiconductor switching element and is widely used in an electronic device.
The basic functions of a DC/DC converter are to turn on and off a switching element at high frequencies, variably control the ratio between on-time and off-time (i.e., a duty ratio), and maintain a DC output voltage at a constant voltage. There are three types of DC/DC converters: a step-up type by which an output voltage higher than an input voltage is obtained; a step-down type by which an output voltage lower than an input voltage is obtained; and a step-down-step-up type by which a constant output voltage is obtained regardless of an input voltage.
A step-down-step-up DC/DC converter has a choke inductor coupled in series or in parallel with a voltage output terminal, and alternates between a state in which energy is stored from the input side in the choke inductor by on-off action of a switching element, and a state in which energy is released from the choke inductor to the output side.
A step-down-step-up DC/DC converter suffers from a larger power loss through switching operations than step-up and step-down types and is thus preferably used when an input voltage and an output voltage are close to each other. In practice, when an output voltage of 3 V is to be outputted, a step-down-step-up DC/DC converter is controlled to operate in a step-down mode when an input voltage supplied from a battery is more than 4 V, operate in a step-down-step-up mode when that the input voltage is more than 2.8 V and not more than 4 V, and operate in a step-up mode when the input voltage is not more than 2.8 V.
Note that an H bridge step-down-step-up DC/DC converter which performs step-down-step-up operation by switching among four states, including state 1 to state 4, is known (see, e.g., Japanese Patent Laid-Open No. 2005-192312).
In a conventional DC/DC converter, the number of switching operations increases with an increase in the number of states in which the DC/DC converter is made to operate. This increases a power loss and reduces the efficiency.